The present disclosure relates to a light source, and particularly a light source such as a light emitting device including an organic light emitting diode (OLED) panel. The disclosure more particularly relates to thermal management issues associated with large area, flexible OLED devices.
OLED devices are generally known in the art and typically include one or more organic light emitting layer(s) disposed between electrodes. For example, the assembly includes a cathode, organic layer, and a light-transmissive anode formed on a substrate so that the assembly emits light when current is applied across the cathode and anode. As a result of the electric current, electrons are injected into the organic layer from the cathode and holes may be injected into the organic layer from the anode. The electrons and holes generally travel through the organic layer until they recombine at a luminescent center, typically an organic molecule or polymer. The recombination process results in the emission of a light photon usually in the ultraviolet or visible region of the electromagnetic spectrum.
The layers of an OLED are typically arranged so that the organic layers are disposed between the cathode and anode layers. As photons of light are generated and emitted, the photons move through the organic layer. Those photons that move toward the cathode, which generally comprises a metal, may be reflected back into the organic layer. Those photons that move through the organic layer to the light-transmissive anode, and finally to the substrate, however, may be emitted from the OLED in the form of light energy. Some cathode materials may be light transmissive, and in some embodiments light may be emitted from the cathode layer, and therefore from the OLED device in a multi-directional manner. Thus, the OLED device has at least cathode, organic, and anode layers. Of course, additional, optional layers may or may not be included in the light source structure.
Cathodes generally comprise a material having a low work function such that a relatively small voltage causes the emission of electrons. Commonly used materials include a wide array of metals, however two commonly used cathode materials include aluminum (Al) and silver (Ag). On the other hand, the anode layer is generally comprised of a material having a high work function value, and these materials are known for use in the anode layer because they are generally light transmissive. Suitable materials include, but are not limited to, transparent conductive oxides such as indium tin oxide (ITO), aluminum doped zinc oxide (AZO), fluorine doped tin oxide (FTO), indium doped zinc oxide, magnesium indium oxide, and nickel tungsten oxide; metals such as gold, aluminum, and nickel; conductive polymers such as poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS); and mixtures and combinations or alloys of any two or more thereof.
Preferably, these light emitting or OLED devices are generally flexible, i.e., are capable of being bent into a shape having a radius of curvature of less than about 10 cm. These light emitting devices are also preferably large-area, which means the OLED devices have a dimensional area greater than or equal to about 10 cm2, and in some instances are coupled together to form a generally flexible, generally planar OLED panel comprised of one or more OLED devices, which has a large surface area of light emission (e.g., on the order of 70 cm2 or greater).
OLED devices operating at 1 watt or greater, but preferably less than 60 W in a large area and generally having a thickness on the order of 800μ (i.e., flexible) or less encounter heating issues at high power. The heating unfortunately results in fast degradation of the OLED device. Consequently, a need exists for improved thermal management in order to increase life and performance.
Thermal management of plastic-based, flexible OLEDs is a particular challenge. It is common practice to use solution based processing techniques for plastic substrates when creating OLED devices, and they tend to not be as efficient as vapor-deposited, glass-substrate OLEDs. Consequently, more of the input power is lost as heat, and thus there is a more demanding need for dissipating that heat via thermal management designs. Additionally, in order to obtain an acceptable shelf life for plastic-based OLEDs, it is common practice to encapsulate the device in a secondary hermetic package. With the flexible OLED structures, dual layer encapsulation is desired in order to obtain the necessary barrier properties that protects against the adverse impact of oxygen and water vapor/moisture. Although the barrier properties are desired, this improved encapsulation creates thermal management issues for the flexible OLEDs. This package can trap the generated heat within the hermetic encapsulation, and so a thermal management scheme is needed for the device as well as the hermetic panel. Lastly, the heat needs to be removed from the panel, and this can be achieved by creating a heat sink to the fixture that contains the OLED panel. Therefore, there are three regions where thermal management designs must be implemented, and these thermal management designs must not adversely impact the flexible nature of the devices.